The vitreous has several significant functions, including the molecular sieve effect, prevention of vasoproliferation, and modification of inflammatory and immunological processes, as shown by recent experimental investigations. However, our understanding of the pathophysiology of vitreoretinal diseases in human is incomplete; only a few chemical, morphological, and immunological studies have been done in pathological human vitreous. The major reason for this is the scarcity of available vitreous specimens. The advent of new vitrectomy procedures provides the unique opportunity to investigate previously discarded human pathological material, which can be obtained in situ during the surgery. This investigation will study pathological vitreous obtained in situ from human eyes undergoing open-sky vitrectomy. It will focus on the clinico-pathological correlation of the vitrectomy material, by use of ophthalmological, chemical, biochemical, tissue culture, immunological, and morphological methods. We will analyze macromolecular composition of pathological vitreous, possible degradation of hyaluronic acid, and regenerative properties of human vitreous. The morphology of pathological vitreous membranes and preretinal membranes will be studied, in order to determine which cells proliferate and produce the membranes. Immunological methods will be applied to study the immunological effect of normal and pathological human vitreous on the stimulation of lymphocytes under tissue culture conditions.